1. Field Of The Invention
The apparatus and method of the present invention relates generally to a drive system for a DC field motor having a rotor and a plurality of stator windings and, more particularly, relates to a phase lock loop commutation position control and method for such a drive system. The present invention has particular applicability for use in a variable speed constant frequency (VSCF) motor/generator system used in an aircraft environment.
2. Description Of The Prior Art
Drive systems and methods for a DC field motor having a rotor and a plurality of stator windings are known. Representative of these are the systems shown in U.S. Pat. No. 3,937,974 and U.S. Pat. No. 3,908,130 to David L. Lafuze, the inventor of the present application, assigned to the General Electric Company, and incorporated by reference herein.
Such drive systems, however, exhibit the following deficiencies. Both systems accomplish phase shifting in accordance with rotation level and/or current level using analog techniques. The range of the possible phase shift using analog techniques is, however, extremely limited. It is difficult to achieve equal phase shifts with respect to each position sensor because the shift is dependent upon the wave amplitude and shape of the output signals of the Hall probes. Mechanical positioning problems with the Hall position probes results in additional position errors, which cannot be readily compensated. Compensation for such position errors requires individual adjustment of the circuitry sensing each probe output, which is difficult to achieve technically and expensive to perform. Hall probes are the contemplated types of position sensors.
Such systems are practically limited to three- or six-phase machines, with six-phase machines being technically equivalent to three-phase machines because of the diametric phase relationships. The analog outputs of the position sensors operate in an open loop system so that performance is highly dependent on the characteristics of the circuit components. No feedback loops are used in such systems to compensate automatically and continuously for component variations and/or the mechanical errors of component positioning. Because no feedback is used, such systems do not employ discriminators. The systems are primarily analog and do not use any type of ring counters or registers to generate timing signals used to control the generation of the thyristor gating signals. Consequently, such systems do not use clocking signal sources, such voltage controlled oscillators, to generate clocking signals for ring counters or shift registers. Such systems employed complex motors with wound rotating fields and means to excite the fields from the stator. Such systems do not use machines employing solid rotors fabricated with permanent magnets, especially permanent magnets of the samarium cobalt type.
Such systems only produce the same number of position signals as the number of position sensors. They do not provide intermediate position signals using either a phase lock loop approach or a phasor addition approach.
Such drive systems have particular application in VSCF motor/generator systems used in aircraft environments. Representative of conventional VSCF power generation systems are the systems disclosed in the following U.S. patents, whose teachings are incorporated herein by reference:
______________________________________ Re 26,630 Peaslee issued 7/15/69 3,152,297 Peaslee issued 10/6/64 3,289,070 Caldwell et al issued 11/29/66 3,320,514 Lawrence issued 5/16/67 3,400,321 Lafuze issued 9/3/68 3,419,785 Lafuze issued 12/31/68 3,593,106 Lafuze issued 7/13/71 3,641,418 Lafuze issued 2/8/72 3,745,471 Lafuze issued 7/10/73 3,873,928 Lafuze issued 3/25/75 3,902,073 Lafuze issued 8/26/75 3,908,161 Messenger issued 9/23/75 ______________________________________